Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Patent
1994-04-26
1996-05-14
Dudash, Diana
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
4272556, 427534, 427536, 427245, B05D 364, C08J 718
Patent
active
055165613
DESCRIPTION:
BRIEF SUMMARY
This invention relates to applying a fluoropolymer film to a body, especially to porous and microporous films of fluoropolymers, and extends to coated bodies and two layer films
Microporous films and membranes from polymers are well known, and asymmetric forms find wide application in filtration and separation. Their manufacture is typically undertaken by a variety of casting processes and other relatively straightforward techniques allowable by the physical and chemical nature of the polymer. Polymers amenable to such straightforward techniques however are thermally, chemically and sometimes physically inferior to the more stable fluoropolymers, e.g. polytetrafluoroethylene (ptfe). Fluoropolymers are selected for their inertness and chemical resistance, and these very properties make it difficult to bond layers of fluoropolymers together. The techniques used for processing ptfe owe more to powder metallurgy than plastics as the material is not a true thermoplastic. The manufacture of such components most usually involves a compression moulding stage and a heat treatment or sintering stage.
PCT Publication NO 90/13593 discloses a mechanical bonding method for porous ptfe layers which are impregnated with perfluoro ion exchange polymer, and further refers to numerous earlier patents in the field. Such a mechanical bond may not be adequate for all applications. Japanese Laid-Open (Kokai) 62-204826 discloses coating a porous ptfe membrance in a plasma vessel by introducing gaseous tetrakis(trifluoromethyl)dithioethane, which forms a polymer in the form of a thin film on the membrane. This introduces sulphur into the product as well as --CF.sub.3 groups at the surface, which is unnecessarily hydrophobic for some applications.
These publications do not teach any way of making a well bonded bilayer of pure fluoropolymer. Such a bilayer could find application in filtration, separation or reverse osmosis.
It would thus be desirable to solve the problem of formation of a very thin continuous layer of fluoropolymer strongly bonded to the surface of a microporous fluoropolymer substrate.
According to the present invention, a method of applying a fluoropolymer film to a body comprises exposing the body to fragments exclusively of the formula --C.sub.n F.sub.2n --, under conditions whereby the fragments combine on the surface of the body to form an adherent fluoropolymer layer. Also according to the invention, a method of applying a fluoropolymer film to a body comprises exposing the body to a supply of saturated molecules of the formula C.sub.n F.sub.2n, causing scission of the molecules, and allowing the fragments to combine on the surface of the body to form an adherent fluoropolymer layer. The body may be carbonaceous polymer e.g. a fluoropolymer such as ptfe, optionally itself a film, which may be porous or microporous, in which case the layer will be covalently bonded thereto. The molecules may be cyclo-perfluoroalkanes e.g. C.sub.n F.sub.2n where n=4-8, preferably 6. The reason for preferring perfluorocycloalkane is that It can undergo scission affording only multiple CF.sub.2 units, in particular no CF.sub.3 fragments at all. This cannot occur with non-cyclic saturated fluoroalkanes. This allows the product to be as close to ptfe, i.e. CF.sub.2 linkages, as possible, avoiding multiple CF.sub.3 fragments, which are more hydrophobic than CF.sub.2. Of the perfluorocycloalkanes, the butane tends to instability, the pentane is possible, while the heptane and octane are becoming exotic for no real gain. The hexane is therefore the most preferred, from cost, stability, availability and volatility points of view.
The body may be etched with a noble gas plasma e.g. argon at say 10-30W, for the purpose of cleaning, before the film is applied. Thereafter, the body may be subjected to a somewhat gentle plasma irradiation, preferably <5W, e.g. 0.1-50W, in a chamber which may be evacuated to 0.01 to 5 torr, such as 0.2 to 0.3 torr, of fluorocarbon. Expressed in terms of unit area-to-be-coated of the body, preferred plasma
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World Patents Index Latest, Week 8528, 3 Jun. 1985, Derwent Publications Ltd., London, GB AN 169068 and JP,A,60 099 326 (Agency Of Ind. Sci. Tech.) 3 Jun. 1985, see abstract.
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Japanese Application No. 62-204826 (English translation).
British Technology Group Ltd.
Dudash Diana
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